Umbriel

Umbriel, third nearest of the five major moons of Uranus and the one having the darkest and oldest surface of the group. Its discovery is attributed to the English astronomer William Lassell in 1851, although the English astronomer William Herschel, who discovered Uranus and its two largest moons, may have glimpsed it more than a half century earlier. Umbriel was named by Herschel’s son, John, for a character in Alexander Pope’s poem The Rape of the Lock.

Umbriel, the third nearest and darkest of Uranus's five major moons, in an image made by Voyager 2 on Jan. 24, 1986. Umbriel is also the most heavily and uniformly cratered of the major Uranian moons, an indicator that its surface experienced little reworking by tectonic activity in the past. The view shows Umbriel's sunlit southern hemisphere. The bright ring near the moon's equator (at the top of the image), dubbed Wunda, is an enigmatic feature that appears to line the floor of an impact crater.NASA/JPL

It orbits Uranus once every 4.144 days at a mean distance of 265,970 km (165,270 miles). Umbriel has a diameter of 1,170 km (727 miles) and a density of about 1.4 grams per cubic cm. The moon appears to be composed of equal parts waterice and rocky material, intermixed with small amounts of frozen methane. (For comparative data about Umbriel and other Uranian satellites, see the table.)

Moons of Uranus

name

mean distance from centre of Uranus (orbital radius; km)

orbital period (sidereal period; Earth days)*

inclination of orbit to planet's equator (degrees)**

eccentricity of orbit

rotation period (Earth days)***

radius (km)

mass (1020 kg)

mean density (g/cm3)

*R following the quantity indicates a retrograde orbit.

**Inclination values in parentheses are relative to the ecliptic.

***Sync. = synchronous rotation; the rotation and orbital periods are the same.

The only images of Umbriel’s surface have come from the U.S. Voyager 2 spacecraft’s flyby encounter with the Uranian system in 1986. These show that Umbriel is distinct from the other major moons of Uranus in having no evidence of past tectonic activity. Its surface is uniformly covered with impact craters, most of them large, measuring 100–200 km (60–120 miles) across. Craters of this size could only have been produced early in the history of the solar system, when planetesimal-size impacting bodies existed. Their presence on Umbriel indicates that the moon’s surface was never subsequently reworked by internal processes. The most notable feature of the hemisphere imaged by Voyager is a bright ring, dubbed Wunda, that appears to line the floor of a crater 40 km (25 miles) across.

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